The present invention relates in general to the field of acoustic emission detection, and in particular, to a new and useful method and apparatus for locating a fluid leakage.
In the past, acoustic emission was used to detect and locate leaks in pressure systems through zonal location techniques, based upon relative signal amplitudes derived from various piezoelectric transducers near the source. Typically, there exist two types of acoustic emissions: burst and continuous. Burst type emission is characterized by acoustic emission signals being generated at a rate slow enough to allow for differentiation between individual events. Acoustic emission source location systems are designed for this type of acoustic emission.
With continuous type emission, the signals become overlapped to the extent that differentiation between the individual events is sometimes impossible. For this reason, it has been believed that when dealing with acoustic emission leak monitoring, one should only concern oneself with steady state parameters such as RMS voltages, because source locations derived from pressure leakages are virtually impossible.
U.S. Pat. No. 4,457,163 to Jackle describes a method and an instrument for precise location of defects in a pipeline by acoustic emissions. The apparatus employs a memory for measurement values, with a frequency analysis and an octave filter. The noise peak of each measurement point as detected by the amplifier is transmitted to digital memory for measurement values which displays a histogram of the noise distribution along the measurement distance along the pipe. In a second process phase, the main and preferred frequency of the loudest measurement point is determined on a broad band and an octave filter is used to determine the characteristic frequency of the emitted medium from the main and preferred frequency as determined on a broad band for the purpose of precise location.
U.S. Pat. No. 4,428,236 to Votava et al describes a method of acoustic emission testing of pipelines for nuclear reactor installations. This reference teaches that the location of a defect can be detected with the aid of the so called triangulation method which is similar to the localization of the center of an earthquake in seismology. In pipelines, a linear orientation or position finding of the defect by measuring the transmission time difference between two test probes is generally sufficient. The method includes subjecting the work piece to a pressure medium such as water and to ultrasonic pulses emitted during deformation development resulting from growth of cracks or from leaks and transmitted through the work piece. These pulses are detected and amplified with equipment formed from these test probes. The test probes are disposed so as to determine respective sources of acoustic emission due to differences in transmission time of the ultrasonic pulses through the component and electronic amplifiers operatively associated with the test probes. The resultant amplified defect signal is then displayed.
Industrial Heating magazine the May issue, 1986, on page 52, discloses that in an acoustic emission signal, the location timing is relative to the time of arrival at the sensors. The time it takes acoustic emission waves to travel to two sensors is used to determine the location of the flaw in the tube.
U.S. Pat. No. 4,609,994 to Bassim et al. discloses an apparatus for continuous long term monitoring of acoustic emission. The apparatus includes a plurality of detector-analyzer units coupled to a central control unit via a communications link. Each detector-analyzer unit includes an acoustic detector, signal processing means, and a microprocessor. The signal processing means may have at least one signal conditioner and one measuring circuit which provides digital output signals representing a set of emission parameters. The microprocessor includes a threshold adjustment means which characterizes incoming data signals as either acoustic emission signals or background noise.
Other techniques which utilize acoustic emissions to determine the presence of leaks or other defects can be found in U.S. Pat. No. 4,013,905 to Breneman et al.; U.S. Pat. No. 4,176,,543 to Nolte et al.; U.S. Pat. No. 4,201,092 to Dau; U.S. Pat. No. 4,410,484 to Marini et al.; and U.S. Pat. No. 4,571,994 to Dickey et al.
The following technical references are also pertinent:
Eisenblatter, J. and Jax, P. "Acoustic Emission Analysis as a Means of Locating Defects and Finding Leaks in Large vessels and Pipework," VGB Kraftwerkstechnick, Vol. 56, No. 7, p. 414-417, July 1976;
Smith, J. R., Rao, D. M., Wassel, W. W. "Advances in Acoustic Leak Monitoring Instrumentation," IEEE
Trans. on Nucl. Sci., Vol. NS-30, No. 1. p. 825-832, Feb. 1983; and
Jax, P. "Flaw Detection and Leak Testing in Components during Internal Pressure Loading with the Help of Acoustic Emission Analysis," Proc. Acoustic Emission Conf., Bad Neuheim, W. Germany, p. 355-383, Apr. 1979.
Despite the presence of various known methods and devices for measuring and evaluating acoustic emissions, and the known applicability of such emissions to detecting the presence of and location for fluid leaks, these are generally either overly complicated or lack accuracy.
In Non-Destructive Testing Handbook, 2nd Ed., published by American Society of Non-Destructive Testing, 1987, at pages 137-144, it is emphasized that when dealing with continuous acoustic sources such as leaks, traditional acoustic emission parameters such as count, count rate, amplitude distribution and conventional delta t measurements, become meaningless and are rarely used. Conventional techniques for measuring time differences between two bursts of acoustic waves cannot be used for continuous sources such as leaks. The conventional and simple measurement of the difference in arrival time between bursts of acoustic emission at two spaced apart sensors is thus not clearly applicable when trying to measure continuous sources of acoustic emission.
While the publication does discuss the use of correlation techniques where two complicated noisy signals are compared to each other to determine their correlation, and thus the time delay between their occurrences, this involves complex computer processing and analysis.